Test repo to figure out cross compilation with carnix

AFAICT individual packages are largely agnostic wrt. what they're being compiled for.
nixpkgs-unstable has neat support for cross compilation without fancy flags:

```
  file $(nix-build --no-out-link '<nixpkgs>' -A pkgsCross.aarch64-multiplatform.hello)/bin/hello
  /nix/store/6924zrj4fww7xs7qzxfidj5n7id6qngi-hello-2.10-aarch64-unknown-linux-gnu/bin/hello: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), dynamically linked, interpreter /nix/store/rcj93w6gk9l6w9jgmqx911iycs2ya9rg-glibc-2.27-aarch64-unknown-linux-gnu/lib/ld-linux-aarch64.so.1, for GNU/Linux 2.6.32, not stripped
```

There *is* a pkgsCross.musl64, but it seems to link musl dynamically, not sure what's up with that.